The present invention is related to a micromechanical acceleration switch of silicon or a similar material.
The invention is suitable for use in systems where one wants to detect when a predetermined acceleration is exceeded. Examples of applications are systems for activating air bags in cars or to turn off systems being subject to strong vibrations, which may be of interest in such different products as CD players, centrifuges and other rotating systems.
Presently, in relation to air bags, acceleration sensors, monitoring the acceleration of a vehicle all the time, and an acceleration switch, giving a signal when the acceleration exceeds a predetermined limit, are being used in combination. The acceleration switches are usually made mechanically so that a spring-loaded mass mounted in a reference system either breaks or makes contact when it is subject to acceleration of certain strength in relation to the reference system. This type of switches are simple, but are relatively large and expensive to manufacture.
Recently micromechanical devices working in a similar way have been described. Examples of these may be found in the patent publications EP-A-056084, U.S. Pat. No. 4,855,544, WO-A-96/21157 and WO-A-85/03383. In these a cantilevered beam is used, optionally with a proof mass element at one end. At a certain acceleration the beam is bent sufficiently to make an electrical contact and thus detect that the acceleration has exceeded a threshold. This threshold level will depend upon the physical parameters of the switch. The devices described in these publications have a number of disadvantages. The beams are provided with electrical conductors, which, because of the differences in thermal and elastic characteristics between the conductors and the material in the beams, make them sensitive to temperature variations. This is, however, mentioned in a somewhat different context in WOA-85/03383. The electrical conductors are also shown with relatively large contact surfaces, which limits the possibility for obtaining reliable operation and a low acceleration threshold level, because the electrostatic pulling force increases with the area.
A common feature for the above-mentioned publications is their sensitivity to vibrations which may cause oscillations of the beams at their resonant frequencies. An attempt to control damping has been made in U.S. Pat. No. 4,855,544 to avoid this by resting the beam against a plate positioned on the opposite side of the beam with respect to the direction of sensitivity. A system exploiting the resonant vibrations is described in EP-A567938, in which a number of beams have been used as described above, each beam being sensitive to different frequencies. In this way the vibration frequency of a system may be detected.
The threshold of the described switches will depend on the dimensions of the beams, and on the proof masses on their ends. This positioning of the proof masses determine, together with the spring constant of the beam, the dynamic range of the switch. When producing different switches with different thresholds, both the length of the beams and the size of the proof masses must be varied and one therefore has little flexibility in choosing the threshold. If a different threshold is wanted, considerable expense is incurred as a new switch will demand new masks and equipment for micromachining the different components. This reduces the possibilities for fine tuning of the switches during production and to limit the production costs, especially in specialised adaptions of components.
An improvement has bee-n described in EP-A-691542 wherein the proof mass is obtained by the asymmetric shape of an electrode element which is suspended by torsion springs attached to the substrate. Both contacting, non-contacting and feedback detection is described.
It is an object of the present invention to provide an acceleration switch which gives reliable and repeatable contact resistance, which is simple and cost effective in production and which may easily be adapted to different applications including use as bi-axial acceleration switches on the same chip.
According to the present invention, there is provided a micromechanical acceleration switch comprising:
an electrode element comprising at least one rod; PA1 a proof mass; PA1 a housing; and PA1 a spring element connecting the electrode element and the proof mass to the housing, wherein the electrode element is mechanically connected adjacent to a proof mass, and wherein the spring element connects the electrode element and the proof mass with the housing so they can pivot, in use, about an axis, wherein:
the at least one rod is flexible, providing a resilient contact between the contact points on the rods and contact areas in the housing; and PA2 the at least one rod has a mass considerably smaller than the proof mass, and is provided with at least one electrical contact point giving electrical contact in co-operation with corresponding contact areas in the switch housing at a predetermined pivot angle of the electrode element, in use; and PA2 the proof mass has its centre of gravity at a given distance from the axis through the spring element.
Generally, the geometrical dimensions of the acceleration switch may be changed based on well-known physical considerations to affect characteristics such as resonant frequency, damping coefficient and threshold, so that it may easily be designed for different applications.
In this way a switch is obtained having a dynamic range given by the size and position of the proof mass, while the threshold may be adjusted by the length of the rod and the distance between the axis through the spring element and the contact points.
Of particular significance for reliable operation is the contact force which needs to be greater than a certain value depending on the electrode materials and their method of fabrication.
In the present invention, the contact force is easily varied by changing the mass and the length of the rod as a function of the dimensions of the proof mass and the spring when the desired acceleration threshold is given. Also, the "sticking" force which opposes the breaking of contact between the electrode surfaces when the acceleration is reduced below the threshold value needs to be taken into account in order to obtain predictable performance. Separating the rod from the proof mass gives freedom in dimensioning this so that the contact force can be varied independently of the dimensions of the proof mass. The threshold of the switch may be made different in the up- and downwards directions by using two rods with different lengths and contact gap, and more than one switch may be made in the same switch housing to give sensitivity to different accelerations in the two directions and/or acceleration in different directions.
The rods may be provided, and may have flexible outer ends providing a resilient contact between the contact points on the rods and the contact areas in the house of the switch. This gives a damped movement which may contribute to avoid unwanted vibrations in the spring-loaded rod ("contact bounce"). Such vibrations may also be damped by filling the switch housing with a damping medium, preferably a viscous gas, and by giving the proof mass a flat shape thus providing an area perpendicular to its direction of movement. In this way the motion of the flat mass is dampened by the surrounding medium by squeeze film damping.
Cross axis sensitivity may be reduced by designing torsional springs with high aspect ratio (height/width/ratio&gt;&gt;1).
Furthermore, the resiliency of the rods gives rise to a wiping motion of the electrode surfaces with respect to one another when the contact opens and closes. This ensures a cleansing effect on the surfaces and contributes to reliable operation of the switch as evidenced by a stable value of contact resistance when the contact is closed even after very many contact operations.
The resilient contact elements allow incorporation of overload protection which limits the motion of the proof mass to pivot angles somewhat greater than the angle at which contact is made with the corresponding contact areas in the housing.
A further advantage of the present invention is that it allows provision of an accelerometer switch for detection of acceleration in a direction substantially in the plane of the substrate or frame. This also implies that one may construct monolithic acceleration switches for detecting acceleration along two orthogonal axes in the plane of the substrate. By this integration of functionality, the switches occupy a minimal area and they are therefore economical to produce and use.